NASA has awarded Northrop Grumman a $400m contract to co-design the Jupiter Icy Moons Orbiter (Jimo) space probe - slated for a rendevouz with Jovian moons Callisto, Europa and Ganymede some time after 2012.

Jimo's mission is to orbit the three satellites for a detailed analysis of each, and in particular investigate the possibility that they have liquid water beneath their frozen surfaces. As is the local custom, scientists believe that such oceans - if present - might harbour primitive life.

The Jimo project has generated a certain amount of controversy due to its uranium-fuelled nuclear fission reactor - which creates electricity to drive the "nuclear electric propulsion (NEP)" system. In simple terms, NEP uses the electricity produced by the reactor to ionise propellant atoms which can then be ejected at high velocity from the vehicle's propulsion system by magnetic or electrified grids. NASA has already proven this "ion drive" technology aboard Deep Space 1, although electricity for the thrusters was in that case provided by solar panels.

Despite safety concerns, NASA reckons that "Space Fission Reactor Power Systems" are the way to go, because they "propel spacecraft directly to the planets in ways not possible today, and perform orbital maneuvers once there; and provide ample electrical power to operate advanced scientific instrument suites that have higher power requirements than instruments used at present".

Jimo's nuclear reactor will be provided by the US Department of Energy's Office of Naval Reactors. Northrop Grumman is involved purely in the vehicle design side, and saw off Boeing and Lockheed Martin to secure the contract. ®

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